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Freeman, Anthony

Publications and source records attributed to Freeman, Anthony.

61 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

A new system model for radar polarimeters

The validity of the 2 x 2 receive R and transmit T model for radar polarimeter systems, first proposed by Zebker et al. (1987), is questioned. The model is found to be invalid for many practical realizations of radar polarimeters, which can lead to significant errors in the calibration of polarimetric radar images. A more general model is put forward, which addresses the system defects which cause the 2 x 2 model to break down. By measuring one simple parameter from a polarimetric active radar calibration (PARC), it is possible to transform the scattering matrix measurements made by a radar polarimeter to a format compatible with a 2 x 2 R and T matrix model. Alternatively, the PARC can be used to verify the validity of the 2 x 2 model for any polarimetric radar system. Recommendations for the use of PARCs in polarimetric calibration and to measure the orientation angle of the horizontal (H) and vertical (V) coordinate system are also presented.

Freeman, Anthony↗
NASA NTRS1990

Calibration experiments with DC-8 SAR data

This paper is a summary of some research activities in calibrating DC-8 SAR data conducted in 1989/90 by the author and others. Firstly, by examining the process of 'symmetrization' in the production of DC-8 Stokes' matrix data, it is shown that an exact solution to the problem of calibrating this data exists, given certain assumptions. Secondly a new, more general, system model for radar polarimeters is proposed, together with a simple approach for verifying the model's validity. The importance of the orientation angle about the radar line of sight for some types of calibration devices is discussed and some results of an experiment to cross-calibrate the C-Band VV data from the DC-8 and DLR E-SAR sensors are presented.

Freeman, Anthony↗
NASA NTRS1990

Polarimetric SAR calibration experiment using active radar calibrators

Active radar calibrators are used to derive both the amplitude and phase characteristics of a multichannel polarimetric SAR from the complex image data. Results are presented from an experiment carried out using the NASA/JPL DC-8 aircraft SAR over a calibration site at Goldstone, California. As part of the experiment, polarimetric active radar calibrators (PARCs) with adjustable polarization signatures were deployed. Experimental results demonstrate that the PARCs can be used to calibrate polarimetric SAR images successfully. Restrictions on the application of the PARC calibration procedure are discussed.

Freeman, Anthony↗
NASA NTRS1989

Phase calibration of polarimetric radar images

The problem of phase calibration between polarization channels of an imaging radar is studied. The causes of various types of phase errors due to the radar system architecture and system imperfections are examined. A simple model is introduced to explain the spatial variation in phase error as being due to a displacement between the phase centers of the vertical and horizontal antennas. It is also shown that channel leakage can cause a spatial variation in phase error. Phase calibration using both point and distributed ground targets is discussed and a method for calibrating phase using only distributed target is verified, subject to certain constraints. Experimental measurements using the NADC/ERIM P-3 synthetic-aperture radar (SAR) system and NASA/JPL DC-8 SAR, which operates at C-, L-, and P-bands, are presented. Both of these systems are multifrequency, polarimetric, airborne, SAR systems.

Sheen, Dan R.↗
NASA NTRS1989

Radiometric Correction and Calibration of SAR Images

A technique for producing radiometrically calibrated SAR image products is described. The output imagery is corrected to represent a measurement of the ground reflectivity or radar cross section. The sources of calibration errors are discussed and the appropriate forms of the radar equation as applied to SAR-image data are reviewed. A key result is the radar equation dependence on the azimuth reference function used in processing. A radiometric correction algorithm for use in an operational SAR correlator is presented. This algorithm has the characteristic that it is fully reversible. Additionally, it can be applied equally to detected or complex SAR images, and it allows for the subtraction of the estimated noise floor in the image but does not require this procedure.

Freeman, Anthony↗
NASA NTRS1989

Approach to derivation of SIR-C science requirements for calibration

A study undertaken to determine SIR-C (Shuttle Imaging Radar-C) science requirements for polarimetric calibration is described. A model describing the effect of miscalibration is presented, followed by a description of specific experiments and the resulting requirements levied on the system. The effects of miscalibration on some commonly used measures which can be extracted from SAR (synthetic aperture radar) data are also discussed. It is concluded that polarimetric calibration requirements are strongly application-dependent, and that a compromise must be made to maximize the domain of application of the data without prohibitive cost. Data summarizing SIR-C polarimetric calibration goals are also presented.

Dubois, Pascale↗